Unless I'm mistaken, the EPC9 is the Aero blowback 9mm. I'm not familiar with the Stearn Defense bolt, but assume it's like any other 9mm bolt cut fir glock and colt mags.
Aero actually sells Endomags for use with their 9mm rigs, so I guess you can say they're supposed to work.
My experience with Endomags has been very positive. My AR9 has gone through many iterations ranging from using glock mags to colt mags and now, Endomags.
I've found the Endomags to be the most reliable. Mine have fed all ball ammunition and most hollow point ammunition, which neither the glock nor colt mags would do reliably.
There are a couple of caveats. First, my AR9 won't feed reliably when it's upside-down, no matter the magazine. The Endo mags do very well with the AR9 rotated 90 degrees to either side, which I had some problems with using other mags. The second thing is that the ejector on the blowback models of Endomags can get damaged which totally screws them up. Endo will replace them.
My current AR9 is a built around a RDB setup and I use the ejectorless endomags. These have been rock solid.